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Filter

The "filter" transform removes rows based on a predicate expression or a selection predicate.

Parameters

Predicate Expression

expr Required
Type: string

An expression string. The row is removed if the expression evaluates to false.

debounce
Type: number

Trailing-edge delay in milliseconds before requesting dataflow replay after a reactive expression dependency changes. Repeated changes restart the delay.

Parameter updates and incoming data are not delayed. Incoming batches use current parameter values, and a completed batch satisfies any pending reactive replay. The delay does not wait for independently scheduled lazy data loads.

Default value: no delay

description
Type: string

A description of the transform step. Can be used for documentation and agent context.

Selection Predicate

param Required
Type: string

A selection parameter. The row is removed if it is not part of the selection.

empty
Type: boolean

If true, the filter retains all rows when the selection is empty.

Default: true

fields
Type: object

An optional mapping of positional channels to fields. Used to determine which fields are checked against the selection intervals.

debounce
Type: number

Trailing-edge delay in milliseconds before requesting dataflow replay after a reactive expression dependency changes. Repeated changes restart the delay.

Parameter updates and incoming data are not delayed. Incoming batches use current parameter values, and a completed batch satisfies any pending reactive replay. The delay does not wait for independently scheduled lazy data loads.

Default value: no delay

description
Type: string

A description of the transform step. Can be used for documentation and agent context.

Example

Filtering by a Predicate Expression

{
  "type": "filter",
  "expr": "datum.p <= 0.05"
}

The example above passes through all rows for which the field p is less than or equal to 0.05.

When a predicate depends on parameters, debounce can delay replay until the dependencies have stopped changing for the specified number of milliseconds. It delays only dependency-triggered replay: parameter updates and new input data remain immediate. A completed input batch uses current parameter values and makes any pending replay unnecessary. The delay does not wait for an independently scheduled lazy load.

Filtering by a Selection Predicate

Interval selections in GenomeSpy are defined by their data extent along the x and/or y channels. When filtering data based on a selection, you must explicitly map the visual channels (x or y) to the corresponding data fields to ensure correct filtering. This ensures that the filter correctly interprets the selection in the context of your dataset.

{
  "description": "Filter transform example using a selection predicate.",

  "data": { "url": "data/sincos.csv" },

  "params": [{ "name": "brush" }],

  "vconcat": [
    {
      "height": 30,
      "transform": [
        { "type": "collect" },
        {
          "type": "filter",
          "param": "brush",
          "fields": { "x": "x", "y": "sin" }
        },
        { "type": "aggregate" },
        {
          "type": "formula",
          "as": "text",
          "expr": "datum.count + ' points selected'"
        }
      ],
      "mark": { "type": "text", "size": 20 },
      "encoding": {
        "text": { "field": "text" }
      }
    },
    {
      "params": [
        {
          "name": "brush",
          "value": { "x": [2.5, 4], "y": [-0.6, 0.6] },
          "select": { "type": "interval", "encodings": ["x", "y"] },
          "push": "outer"
        }
      ],
      "mark": { "type": "point", "size": 100 },
      "encoding": {
        "x": { "field": "x", "type": "quantitative" },
        "y": { "field": "sin", "type": "quantitative" }
      }
    }
  ]
}